1. Field of the Invention
The present invention relates a fuel cell system including a fuel cell having an oxygen-containing gas flow field for supplying an oxygen-containing gas to a cathode and a fuel gas flow field for supplying a fuel gas to an anode to generate electricity by electrochemical reactions of the oxygen-containing gas and the fuel gas. Further, the present invention relates to a method of stopping operation of the fuel cell system, and starting operation of the fuel cell system.
2. Description of the Related Art
Fuel cells are systems for obtaining direct current electrical energy by supplying a fuel gas (chiefly containing hydrogen) and an oxygen-containing gas (chiefly containing oxygen) to an anode and a cathode for inducing electrochemical reactions at the anode and the cathode.
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which includes a pair of electrodes (anode and cathode), and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly is interposed between a pair of separators. The membrane electrode assembly and the separators make up a unit of power generation cell for generating electricity. Generally, in the power generation cell of this type, predetermined numbers of membrane electrode assemblies and separators are stacked alternately to form a fuel cell stack.
In the fuel cell of this type, when power generation (operation) is stopped, though supply of the fuel gas and the oxygen-containing gas to the fuel cell is stopped, some fuel gas remains at the anode, and some oxygen-containing gas remains at the cathode. Therefore, during the stop of operation of the fuel cell, the cathode is kept to have a high potential, and degradation of the electrode catalyst layer occurs.
In this regard, for example, an approach for forcibly purging the fuel gas remaining at the anode using an inert gas such as the air, nitrogen, or the like is adopted. Therefore, when operation of the fuel cell stack is stopped, for example, the air is present at the cathode and the anode.
Further, even if the above purge process is not carried out, in the case where operation of the fuel cell stack has been stopped for a long period of time, the air passes through the electrolyte membrane from the cathode toward the anode. Therefore, the air is present at both of the cathode and the anode.
In this state, if operation of the fuel cell is started, at the time of starting supply of the fuel gas to the anode, since hydrogen and the air are mixedly present, the cathode tends to have a high voltage. Thus, the power generation performance may be degraded undesirably due to degradation in the performance of the electrode catalyst layer of the cathode (Japanese Laid-Open Patent Publication No. 2006-507647 (PCT)).
In an attempt to address the problem, for example, in a method of stopping a fuel cell power plant disclosed in U.S. Pat. No. 7,141,324, a load is disconnected from a fuel cell, and supply of an oxygen-containing gas to an oxygen-containing gas flow field is stopped, and then, a fuel gas is supplied to the oxygen-containing gas flow field for filling the oxygen-containing gas flow field and the fuel gas flow field with the fuel gas.
However, in the above conventional technique, at the time of stopping operation of the fuel cell, it takes relatively long time to replace the oxygen-containing gas remaining in the oxygen-containing gas flow field with the fuel gas. Therefore, it takes considerable time to completely stop operation of the fuel cell, and the process of stopping operation of the fuel cell cannot be performed efficiently.